Treatment of human cancers with chemotherapy or radiation with curative intent has led to the successful eradication of malignancies in millions of patients, yet the apoptotic cell death induced in healthy tissues drastically limits the use of thee crucial therapies. This is especially true in pediatric patients that, for example, commonly experience cardiotoxicity from doxorubicin treatment or neuronal apoptosis after brain irradiation. Adults exhibit dramatically less toxicity than children from these same treatments but the basis for this difference in sensitivity is unknown. Using BH3 profiling, an innovative tool to measure how close cells are to the threshold of apoptosis, we recently observed novel and striking differences in the how apoptosis is regulated in vital tissues that challenge the existing dogma in the field. These observations also create opportunities to improve existing therapies and develop novel classes of anti-cancer drugs. Within this proposal, we plan to first develop a comprehensive understanding of how differential regulation of apoptosis affects cell fate in response to damage or stress in vivo (Aim 1). Using gene expression analysis and mouse models, we will then identify the molecular mechanisms that control these pathways (Aim 2). Finally, we will utilize our newfound knowledge to identify and develop agents that will reduce toxicity from current treatments or represent novel classes of anti-cancer therapies (Aim 3). By understanding and modulating apoptosis programs in healthy and cancerous cells we will improve patient outcomes.